In recent years, there has been much emphasis in the automotive industry on the introduction of equipment for increasing the safety of vehicles. An example of such equipment is an airbag system, which causes an airbag to be inflated with gas between passengers and the steering wheel, the dashboard, or other portions of the vehicle interior before the passengers can impact therewith at the time of a collision and thereby absorbs the kinetic energy of the passengers so as to decrease the likelihood and severity of injuries.
A conventional airbag system employs an explosive chemical to generate a gas to inflate an airbag. However, due to a desire for a quick response to airbag inflation and for adjustment of the pressure of an inflating gas, another type of airbag system which employs a gas accumulated in a steel pipe under high pressure to inflate an airbag has been developed and is being increasingly employed.
An airbag system of the latter type normally maintains an inflating gas at a high pressure in an accumulator. When a collision occurs, the inflating gas in the accumulator is discharged all at once into an airbag. As a result, a steel pipe which is used as an accumulator for the high pressure inflating gas undergoes stress in an extremely short period of time at a high strain rate. Therefore, in contrast to a simple structural member such as a conventional pressure cylinder or line pipe, the steel pipe used as the inflating gas accumulator in such an airbag system is required to have high dimensional accuracy, workability, and weldability, and it is also required to have high strength and excellent resistance to bursting.
A steel pipe which is suitable for use with an airbag system to form an inflating gas accumulator and a method for its manufacture are described in the following published Japanese patent applications: JP P10-140238A, JP P10-140249A, JP P10-140250A, JP P10-140283A, JP P10-212549A, JP P2002-294339A, JP P11-19929A, JP P2001-49343A, and JP P2002-194501A.
Although the above-listed patent applications aim at providing a high strength, high toughness steel pipe suitable for use with an airbag system, the target tensile strength described in those applications is on the level of 590 MPa or above, and the values of tensile strength obtained in the examples of those applications are no higher than 947 MPa.
The steel pipes disclosed in the above patent applications can provide adequate performance for existing airbag systems. However, as a result of the trend in recent years towards decreases in the weight of automobiles, there has also come to be a demand for decreases in the size and weight of airbag systems. Accordingly, there is a demand for steel pipes for airbag systems to be able to accumulate an inflating gas at a higher pressure while having a smaller wall thickness.